bits varnish: serve 204s for /statsv

statsv is a logging beacon endpoint for processing client-side statsd-like
performance metrics, generated by MediaWiki's JavaScript code.

* Make //bits.wikimedia.org/statsv/* reqs HTTP 204s, like EventLogging's
  /event.gif endpoint.
* Fix the RxURL arg for the Varnishkafka instance to not require an additional
  slash.
* Add a comment explaining both endpoints.

Change-Id: I6774c0b6508879226dfb41fb123c9ac4dfe132d3
Merge "bits varnish: serve 204s for /statsv" into production